Boiler

ABSTRACT

A boiler has a combustion chamber formed by front and rear walls and a side wall extending between the front and rear walls. Plural stages of burners are placed on at least one of the front and rear walls. In the front and rear walls are opposing gas jets for making a pressure of the gas near the side wall within the combustion chamber higher than the pressure of the gas at a center portion of the combustion chamber. The gas jet ports are disposed at a height within a range of the height of the burner stages. The burner stages supply the pulverized coal, the air for transferring the pulverized coal and the air for burning. A part of the air for transferring the pulverized coal or the air for burning is supplied in a branched manner to the gas jet port and injected into the combustion chamber. Further, the air is preferably injected from the gas jet ports in a direct gas flow, not a swirling flow.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a boiler, and more particularly, to aboiler which is preferable for reducing a concentration of CO, anunburned matter, an attached ash and the like near a side wall of afurnace.

2. Description of the Prior Art

In order to improve a heat efficiency of a boiler, it is necessary toreduce a concentration of a carbon monoxide (CO) and an unburned matterwithin a furnace. In order to reduce the concentration of CO and theunburned matter within the furnace, there has been known the followingmethod.

A first method corresponds to a method of adjusting an operationcondition, in particular, a method of adjusting an air flow amount in aburner and an air flow amount in an after air port for a two stagecombustion.

A second method corresponds to a method of supplying an air to a spacein which an unburned matter is increased. As an example of the secondmethod, a method of supplying an air along a wall of a furnace is shownin Japanese Utility Model Unexamined Publication Nos. 59-92346 and2-122909, and Japanese Patent Unexamined Publication Nos. 62-131106 and3-286918.

Among these conventional examples, in Japanese Utility Model UnexaminedPublication Nos. 59-92346 and 2-122909, and Japanese Patent UnexaminedPublication No. 3-286918, there is disclosed a boiler in which an airport is provided in a lower portion of a burner stage.

In Japanese Patent Unexamined Publication No. 62-131106, there isdisclosed a boiler in which the air ports are provided on four walls ofthe furnace and the air ports are provided on upper and lower portionsand an intermediate height of a plurality of burner stages.

Inventors have verified an effectiveness of the conventional first andsecond methods mentioned above on the basis of a measurement and anumerical analysis of an actual boiler. As a result, it has becomeapparent that the concentration of CO and the unburned matter in thecombustion gas have still been high near the side wall(s) extendingbetween the front and rear wall having the burner walls having theburner(s), at least at a height of the burner stage, even when any ofthese methods is employed. Further, it has become apparent that the ashis attached to the side wall in the case of the burning of coal.

The reason is that the combustion gas generated from the burner comesnear the side wall adjacent to the wall having the burner since thepressure near the side wall is lower than that of the combustion area atthe center of the furnace.

A countermeasure thereof is shown in Japanese Patent UnexaminedPublication No. 7-98103. In this example, there is suggested a boilercomprising a plurality of burners and a plurality of air inlet ports fora two stage combustion disposed downstream of the burners, which isstructured such that an auxiliary combustion port for supplying a gasfor combustion having an oxygen partial pressure of 10% or less isprovided between a side wall of a furnace and a burner so as to adjustan injection amount of the gas for combustion injected from theauxiliary combustion port and a direction of a jet, thereby preventing aburner jet from returning to the side wall of the furnace.

However, in this prior art, a pipe for supplying the gas for combustionhaving the oxygen partial pressure of 10% or less to the auxiliarycombustion port is required. Since it is necessary to arrange a pipe forsupplying the gas for combustion having a length of about some tens ofmeters, a great cost increase can not be avoided.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a boiler structuredsuch as to prevent a combustion gas from coming near a side wall byusing air, oxygen, combustion exhaust gas and the like.

The present invention provides a boiler comprising a combustion chamberformed by front and rear walls and a side wall between to said front andrear walls and a plural stages of burners placed on at least one of saidfront and rear walls, in which in order to make a pressure of a gaswithin said combustion chamber higher in a portion near the side wallthan at a center portion of said combustion chamber, a gas port isprovided between an outermost row burner and said side wall within arange of a height of said burner stages.

The present invention also provides a boiler comprising a combustionchamber formed by front and rear walls and a side wall between saidfront and rear walls and plural stages of burners placed on at least oneof said front and rear walls, in which in order to make a pressure of agas near said side wall within said combustion chamber higher than apressure of a gas at a center portion of said combustion chamber, a gasjet port is provided in said side wall within a range of a height ofsaid burner stages.

The present invention further provides a boiler comprising a combustionchamber formed by front and rear walls and a side wall between saidfront and rear walls, plural stages of burners placed on at least one ofsaid front and rear walls and an after air port for two stage combustiondisposed downstream of said burner stages, wherein at least one stagegas jet port for making a pressure of a gas near said side wall withinsaid combustion chamber higher than a pressure of a gas at a centerportion of said combustion chamber is provided between an outermost rowburner and said side wall within a range of a height of said burnerstages and plural stages of gas jet ports are provided between saidlowermost stage burner and said after air port.

In each of the boilers mentioned above, it is desirable that said gasport is provided at portions of said opposing front and rear walls, saidportions having the same height, and wherein gas supply means forinjecting said jet at a speed at which a gas jet from said opposing gasport collides in the middle of said front and rear walls is provided.

The present invention, more particularly, provides a boiler as cited inany one of the structures mentioned above, comprising supply means forsupplying a pulverized coal as a fuel and air for transferring saidpulverized coal to said plural stages of burners, and supply means forsupplying-an air for combustion to said plural stages of burners andsupply means for supplying a gas for jetting to said gas port, in whichthere is provided control means for controlling a flow amount of the jetfrom said gas port on the basis of a load demand of said boiler andinformation of the coal type information so as to reduce a flow amountof the jet from said gas port when a load of said boiler is low and toincrease the flow amount of the jet from said gas port when the load ofsaid boiler becomes higher.

The present invention further provides a boiler as cited in any one ofthe structures mentioned above, comprising supply means for supplying apulverized coal as a fuel and air for transferring said pulverized coalto said plural stages of burners, and supply means for supplying an airfor combustion to said plural stages of burners and supply means forsupplying a gas for jetting to said gas port, in which measurement meansfor measuring a concentration of a carbon monoxide (CO) in a combustiongas near said side wall is provided, and there is provided control meansfor controlling a flow amount of the jet from said gas port on the basisof the load demand of said boiler and a measured result of saidconcentration of CO so as to reduce the flow amount of the jet from saidgas port when a load of said boiler is low, increase the flow amount ofthe jet from said gas port when the load of said boiler becomes higherand reduce a flow amount of the jet from said gas port when saidconcentration of CO is equal to or less than the predetermined value.

The control means may be means for increasing the flow amount of saidjet in accordance with a decrease in the fuel ratio of the pulverizedcoal.

The supply means for supplying the gas for jetting to said gas port maybe means for branching the air for combustion of said burner so as tomake the air for jetting. In this case, it is preferable that a flowamount adjusting damper is provided in each of a flow passage of the airfor combustion and a flow passage of the air for jetting.

The supply means for supplying the gas for jetting to said gas port maybe means for branching the air for transferring said pulverized coal soas to make the air for jetting.

In the case that an after air port for a two stage combustion is placeddownstream of said burner stage, the supply means for supplying the gasfor jetting to said gas port can be means for branching the after air soas to make the air for jetting.

In accordance with the present invention, since in a boiler comprising acombustion chamber formed by front and rear walls and a side wallbetween said front and rear walls and plural stages of burners placed onat least one of said front and rear walls, in order to make a pressureof a gas within said combustion chamber higher in a portion near theside wall than at a center portion of said combustion chamber, a gasport is provided between an outermost row burner and said side wallwithin a range of a height of said burner stages, and it is possible toincrease a pressure of the gas near the side wall so as to prevent thecombustion gas from coming close to the side wall, thereby reducing anattachment of the ash due to a collision of the combustion gas, aconcentration of CO at an outlet of the combustion chamber and anunburned matter.

In this case, in the embodiments which will be mentioned below, a boilercorresponds to a boiler in which a combustion gas generated by acombustion of a fuel flows from an inlet port of a fuel toward an outletport of a furnace in one direction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view which shows a summarized structure of afurnace in an embodiment 1 of a once-through boiler in accordance withthe present invention;

FIG. 2 is a cross sectional view which shows an embodiment of astructure of a gas port in the embodiment 1;

FIG. 3 is a front elevational view which shows an embodiment of astructure of the gas port in FIG. 2;

FIG. 4 is a view which shows a summary of a stream of a combustion gasand a gas jet within the furnace in the embodiment 1 in which the gasport is placed on a front wall and a rear wall;

FIG. 5 is a front elevational view which shows a summary of a stream ofthe combustion gas in the conventional furnace in which the gas port isnot placed;

FIG. 6 is a view which shows a summary of a stream of a combustion gasand a gas jet within a furnace in accordance with an embodiment 2 inwhich a gas port is placed on a left side wall and a right side wall;

FIG. 7 is a perspective view which shows a summarized structure of afurnace in an embodiment 3 of a once-through boiler in accordance withthe present invention;

FIG. 8 is a front elevational view which shows a stream line toward adirection of the left side wall in the embodiment 3;

FIG. 9 is a view which shows a result of calculating a concentration ofCO (%) at a position 10 cm apart from the left side wall of theembodiment 3;

FIG. 10 is a front elevational view which shows a stream line toward adirection of the left side wall in the conventional once-through boiler;

FIG. 11 is a view which shows a result of calculating a concentration ofCO (%) at a position 10 cm apart from the left side wall shown in FIG.10;

FIG. 12 is a front elevational view which shows a stream line toward adirection of the left side wall in the conventional once-through boilerin which an apparatus of an air flowed near boundary layer of wall forforming a stream of an air along the wall is provided at a lower portionof the furnace;

FIG. 13 is a view which shows a result of calculating a concentration ofCO (%) at a position 10 cm apart from the left side wall shown in FIG.12;

FIG. 14 is a view which shows a comparison of characteristic between aburner A in which a stoichiometric ratio is near 0.8 and a value ofNitrogen Oxide at the outlet of the furnace becomes a minimum value anda burner B in which a stoichiometric ratio is near 0.7 and a value ofNitrogen Oxide at the outlet of the furnace becomes a minimum value;

FIG. 15 is a systematic view which shows a structure of an embodiment 4of a once-through boiler in accordance with the present invention;

FIG. 16 is a characteristic view which shows an embodiment of a relationbetween a load and a flow amount of a jet from the gas port;

FIG. 17 is a characteristic view which shows an embodiment of a relationbetween a fuel ratio and a flow amount of a jet from the gas port;

FIG. 18 is a characteristic view which shows an embodiment of a relationbetween a concentration of CO and a flow amount of a jet from the gasport;

FIG. 19 is a side elevational view of a furnace which shows supply meansfor supplying an air for jet by branching an air for combustion in aburner;

FIG. 20 is a side elevational view of a furnace which shows supply meansfor supplying an air for jet by branching from an upstream of a damperfor adjusting an air flow amount in the burner; and

FIG. 21 is a side elevational view of a furnace which shows anembodiment in which in the case that the gas port is close to the afterairport, the air for jet is branched from the after air and the air pipeis made shorter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, embodiments of a once-through boiler in accordance with thepresent invention will be described below with reference to FIGS. 1 to21.

Embodiment 1

FIG. 1 is a perspective view which shows a summarized structure of afurnace in an embodiment 1 of a once-through boiler in accordance withthe present invention. The furnace has a front wall 14 a and a rear wall14 b, and a left side wall 1 a and a right side wall 1 b crossing to orextending between the walls 14 a and 14 b. A plurality of burners aremounted to at least one of the opposing front wall 14 a and rear wall 14b in plural stages and plural rows. In the case of the embodiment 1, alower stage burner 2, a middle stage burner 3 and an upper stage burner4 are respectively constituted by four rows of burners. Each of theburners supplies a fuel and an air for combustion to a combustionchamber 13.

A gas port 6 arranged by the present invention is positioned between thelower stage burner 2 and the upper stage burner 4 in a height directionand between a side wall 1 and an outermost row burner in a lateraldirection. The gas port 6 in the embodiment 1 is formed at a portionhaving the same height as that of the middle stage burner 3. The gasport 6 of the front wall 14 a and the gas port 6 of the rear wall 14 bare formed at a position at which a jet of the gas port 6 collides.

In the embodiment 1, a gas not containing a fuel is supplied from thegas port 6. A component of the gas not containing the fuel includes air,oxygen, burned exhaust gas and the like. It is not necessary that flowspeeds of the opposing jets are equal to each other, and it is possibleto adjust the position at which the jets collide with each other and thepressure in the colliding position when changing the flow speed and theflow amount of the jets.

FIG. 2 is a cross sectional view which shows an embodiment of astructure of the gas port 6 in accordance with the embodiment 1. FIG. 3is a front elevational view which shows the embodiment of the structureof the gas port 6 shown in FIG. 2. A shape of the gas port 6 is definedby a water tube 17 constituting a boiler. The water tube 17 is arrangedaround the gas port 6 in a direction parallel to a center axis of thegas port 6. When arranging the water tube 17 in this manner, a dampingof a jet 18 in the gas port 6 is reduced so as to increase a pressure ata time when the jet 18 is collided. An optimum shape of the gas port 6is a cylindrical shape in which a cross section is a circular shape.When the cross section of the gas port 6 is a circular shape, it is easyto bend the water tube 17 so as to form the gas port 6.

FIG. 4 is a view which shows a summary of a stream of the combustion gas16 and the jet 18 within the furnace of the embodiment 1 in which thegas port 6 is placed in the front wall 14 a and the rear wall 14 b. Whenplacing the gas port 6, the combustion gas 16 can not come close to theside walls 1 a and 1 b due to the jet 18 of the gas injected from thegas port 6. Because the pressure near the side walls 1 a and 1 b becomesincreased due to the jet 18 of the gas injected from the gas port 6.

FIG. 5 is a front elevational view which shows a summary of a stream ofthe combustion gas in the conventional furnace in which the gas port 6is not placed. In the case that the gas port 6 is not placed, thecombustion gas 16 formed by the burner stages 2, 3 and 4 flows in thedirection of the side walls 1 a and 1 b. Since the combustion gas 16from the lower stage burner 2 is prevented by the combustion gas 16 inthe middle burner 3 and the upper burner 4 and can not ascend in animmediately upper direction, the gas 16 flows in a direction of the sidewalls 1 a and 1 b in which a pressure is low.

A certain effect can be obtained even when the gas port 6 is formedbetween a bottom of the furnace to a top thereof not immediately besidethe burner stages 2, 3 and 4. However, the effect becomes small when itis placed at a portion apart from the burner stages 2, 3 and 4.

As shown in the prior art, when forming the gas port 6 in a lower sideof the burner stages 2, 3 and 4, the pressure of the portion near theside wall 1 becomes high at the formed height, however, the pressurebecomes low at the height of the burner stages 2, 3 and 4, so that thecombustion gas 16 generated by the burners 2, 3 and 4 flows in adirection of the side walls 1 a and 1 b.

When forming the gas port 6 in an upper side of the burner stages 2, 3and 4, the pressure of the portion near the side walls 1 a and 1 b isincreased in comparison with the case that the gas port 6 is not formed.However, in comparison with the case of forming the gas port in the areaof the burner stages 2, 3 and 4, an increase of the pressure is a littleand the combustion gas 16 generated by the burners 2, 3 and 4 easilyflows in a direction of the side walls 1 a and 1 b.

The jet 18 from the gas port 6 can achieve the object of the presentinvention well when reaching the center portion of each of the sidewalls 1 a and 1 b. In the case that the jet 18 can not reach the centerportion of each of the side walls 1 a and 1 b, the combustion gas 16easily flows in a direction of the side walls 1 a and 1 b. Accordingly,it is necessary to collide the jet 18 in the center portion of each ofthe side walls 1 a and 1 b. A desirable flow speed of the jet 18 iswithin a range between 30 m/s and 90 m/s. Further, in the case that thegas port 6 is of the type of supplying a direct flow gas, since it ispossible to make the damping of momentum of the gas smaller than thetype of supplying a swirling flow gas, it is possible to supply the gasto the center portion of the side walls 1 a and 1 b at a higherpressure.

The jet 18 at the gas port 6 maybe not only supplied in perpendicular tothe burner wall 14 but also supplied at an optional angle. Whensupplying the jet 18 at the gas port 6 in such a manner as to direct toan inner portion of the combustion chamber 13, it is hard that thecombustion gas 16 flows in a direction of the side wall 1. Wheninjecting the jet 18 toward the side wall 1, the gas in the jet 18 canbe supplied along the side wall 18. When the combustion gas 16 comesclose to the side wall 1, a heat absorption of the side wall 1 isincreased, so that a temperature of a water wall constituting the sidewall 1 is increased. The jet 18 at the gas port 6 also serves to coolthe side wall 1.

Embodiment 2

FIG. 6 is a view which shows a summary of a stream of the combustion gas16 and the jet 18 within the furnace in accordance with an embodiment 2in which a gas port 8 is placed on the left side wall 1 a and the rightside wall 1 b. The structures of the burner stages 2, 3 and 4, the frontwall 14 a and the rear wall 14 b are the same as those of theembodiment 1. It is not necessary that the gas port 8 is placed only onthe front wall 14 a and the rear wall 14 b on which the burner stages 2,3 and 4 are arranged. When the gas port 8 is placed on the side walls 1a and 1 b, the same effect as that of the embodiment 1 can be obtained.In this case, as well as the embodiment 1, it is necessary to increasethe pressure near the side walls 1 a and 1 b. It is proper to set a flowspeed of the jet 18 to a range between 30 m/s and 90 m/s. Further, thejet 18 at the gas port 8 may be directed not only perpendicularly to therear wall 14 b but also at an optional angle. FIG. 6 shows an embodimentin which the jet 18 is supplied downward. When directing the jet 18downward, the jet 18 and the combustion gas 16 are collided with eachother, so that the pressure is increased. As a result, the combustiongas 16 can not come close to a direction of the side walls 1 a and 1 b.

Embodiment 3

FIG. 7 is a perspective view which shows a summarized structure of afurnace in an embodiment 3 of a once-through boiler in accordance withthe present invention. The structures of the burner stages 2, 3 and 4,the front wall 14 a and the rear wall 14 b are the same as those of theembodiment 1. An after air port 9 for a two stage combustion is mountedto an upper portion of the burner stages 2, 3 and 4. At least one stageof gas port 6 is placed between the lower stage burner 2 and the upperstage burner 4, and plural stages of gas ports 6 are placed between thelower stage burner and the after air port 9. In the embodiment 3, theyare mounted at a portion having the same height as that of the middleburner 3, between the upper burner 4 and the after air port 9 and aportion having the same height as that of the after air port 9.

The jet 18 from the gas port 6 increases the pressure at the centerportion of the side wall 1 and prevents the combustion gas 16 fromcoming close to the side wall 1, as in the same manner as that of theembodiment 1. When placing the gas port 6 in the burner stages 2, 3 and4, it is hard that the combustion gas 16 comes close to the side walls 1a and 1 b and at the same time the deoxidization gas generated inaccordance with a two stage combustion method is oxidized, so that it ispossible to reduce the concentration of CO and the unburned matter nearthe side wall 1 a and 1 b. Further, the pressure near the side walls 1 aand 1 b is increased by placing a plural stages of gas ports 6 as shownin FIG. 7, so that it is hard that the combustion gas 16 containing thedeoxidization gas comes close to the side wall 1.

FIG. 8 is a front elevational view which shows a stream line 41 toward adirection of the left side wall 1 a in the embodiment 3. FIG. 9 is aview which shows a result of calculating a concentration of CO (%) at aposition of 10 cm apart from the left side wall 1 a in the embodiment 1.The numerically analyzed boiler is a boiler having a maximum outputpower of a pulverized coal flame of 500 MW and under a state of 100%load. 4% air for combustion is supplied from the gas port 6. Aninjection speed is 40 m/s.

FIG. 10 is a front elevational view which shows the stream line 41toward a direction of the left side wall 1 a in the conventionalonce-through boiler. FIG. 11 is a view which shows a result ofcalculating a concentration of CO (%) at a position of 10 cm apart fromthe left side wall 1 a shown in FIG. 10. The numerically analyzed boileris a boiler having a maximum output power of a pulverized coal flame of500 MW and under a state of 100% load.

FIG. 12 is a front elevational view which shows a stream line 41 towarda direction of the left side wall 1 a in the conventional once-throughboiler in which an apparatus of an air flowed near boundary layer ofwall 42 for forming an air flow along the wall is provided in the lowerportion of the furnace. FIG. 13 is a view which shows a result ofcalculating a concentration of CO (%) at a position of 10 cm apart fromthe left side wall 1 a shown in FIG. 12. The numerically analyzed boileris a boiler having a maximum output power of a pulverized coal flame of500 MW and under a state of 100% load. 8% air for combustion is suppliedfrom the apparatus of an air flowed near boundary layer of wall 42 shownin FIG. 12 as an air flowed near boundary layer of wall 43.

In comparison among FIGS. 8, 10 and 12, in the case that the gas port 6is provided in such a manner as shown in FIG. 8 on the basis of theembodiment 1 of the present invention, the flow of the combustion gas 16toward a direction of the side walls 1 a and 1 b is less than theconventional embodiment shown in FIG. 12. In particular, there hardlyexists the stream toward a direction of the side walls 1 a and 1 b fromthe middle stage burner 3 and the upper stage burner 4. The jet 18 fromthe gas port 6 prevents the combustion gas 16 from colliding with theside wall. In the case that the apparatus of an air flowed near boundarylayer of wall 42 shown in FIG. 12 is provided, it is hardly possible toprevent the combustion gas 16 from colliding with the side walls 1 a and1 b.

A concentration of CO near the side wall 1 in the embodiment 1 inaccordance with the present invention shown in FIG. 9 becomes equal toor less than 1% at a portion downstream the burner stage.

A concentration of CO near the side wall 1 in the conventional typeboiler shown in FIG. 11 attains 10% at the maximum between the upperstage burner 4 and the after air port 9. Carbon monoxide near the sidewall 1 is hard to be oxidized and flows to the outlet port 5 of thefurnace.

A concentration of CO near the side wall 1 in the conventional typeboiler in which the air flowed near boundary layer of wall 42 shown inFIG. 13 is placed is 8% at the maximum, and is hardly different fromthat of the conventional type boiler. The distribution of theconcentration of CO mentioned above is established because thecombustion gas 16 flowing from the burners 2, 3 and 4 flows in adirection of the side wall 1 having a low pressure even after flowingthe air flowed near boundary layer of wall 42 along the side wall 1,thereby colliding with the side wall 1.

FIG. 14 is a view which shows a comparison of characteristic between aburner A in which a stoichiometric ratio of fuel is near 0.8 and a valueof Nitrogen Oxide at the outlet 5 of the furnace becomes a minimum valueand a burner B in which a stoichiometric ratio of burner is near 0.7 anda value of Nitrogen Oxide at the outlet 5 of the furnace becomes aminimum value. It is desirable that the burner used in the embodiment 3has a characteristic that a value of Nitrogen Oxide at the outlet 5 ofthe furnace becomes a minimum value under an operation condition suchthat a stoichiometric ratio of burner is lower than 0.8. When using theburner B, in order to reduce Nitrogen Oxide at the outlet 5 of thefurnace, it is effective to reduce the stoichiometric ratio of 0.7rather than 0.8. However, when lowering the stoichiometric ratio of, thedeoxidization gas generated in the burner stages 2, 3 and 4 flows nearthe side wall 1, thereby increasing the concentration of CO and theunburned matter.

Accordingly, the conventional type boiler has been operated under acondition of the stoichiometric ratio of about 0.8, and Nitrogen Oxideat the outlet 5 of the furnace has been substantially the same betweenthe burner A and the burner B.

On the contrary, in accordance with the present invention, for example,since it is possible to reduce the concentration of CO and the unburnedmatter near the side wall 1 when placing the gas port 6 as shown in theembodiment 3, it is possible to use the burner B in which thestoichiometric ratio is near 0.7 and a value of Nitrogen Oxide at theoutlet 5 of the furnace becomes a minimum value, so that in comparisonwith the case of using the burner A, it is possible to reduce NitrogenOxide at the outlet S of the furnace.

Embodiment 4

FIG. 15 is a systematic view which shows a structure of an embodiment 4of a once-through boiler in accordance with the present invention. Aused fuel is a coal 23 and is stored in a coal bunker 37. A coal storedin the coal bunker 37 is pulverized by a coal pulverizer 38. A coalfeeding air 33 and the coal are supplied to a burner 39. An air suppliedfrom a blower 31 is heated by a burned exhaust gas 32 and an air heater30. The heated air is separated into a coal feeding air 34, an air forcombustion 35 and an air for jet 36 at the gas port 6. A damper 27 and aflow amount meter 26 are placed in pipes for the coal feeding air 34,the air for combustion 35 and the air for jet 36. A control apparatus 20inputs a load demand 21, a coal type information 22, a coal typemeasuring result 24 and a flow amount signal 25 of the air for jet 36 soas to control a damper 27 of the air for jet 36. It is sufficient thatthe gas port 6 is placed in such a manner as shown in the embodiment 1or the embodiment 2.

The control apparatus 20 estimates a characteristic of the coal on thebasis of the coal type information 22 or the coal type measuring result24, controls an opening degree of the damper 27 in response to theestimated coal characteristic, the load demand and the flow amount 25 ofthe air for jet 36, and adjusts a jet 18 from the gas port 6.

FIG. 16 is a characteristic view which shows an example of a relationbetween the load and the flow amount of the jet 18 from the gas port 6.Since the pressure of the combustion area within the furnace is not highwhen the load is low, a flow amount of the combustion gas 16 flowing ina direction of the side wall 1 is a little. Accordingly, the flow amountof the jet 18 from the gas port 6 is set to be a little. As the loadbecomes higher, the flow mount of the jet 18 from the gas port 6 is setto be increased.

FIG. 17 is a characteristic view which shows an example of a relationbetween a fuel ratio and the flow amount of the jet 18 from the gas port6. In the case of a coal having a low fuel ratio, since an amount of adeoxidization gas in the combustion gas 16 flowing in a direction of theside wall 1 is increased, the flow amount of the jet 18 from the gasport 6 is set to be increased. On the contrary, in the case of a coalhaving a high fuel ratio, since a combustion is not promoted and theamount of the deoxidization gas is reduced in comparison with the coalhaving a low fuel ratio, the flow amount of the jet 18 from the gas port6 is set to be reduced.

When setting the flow amount of the jet 18 at the gas port 6 to beminimum without breaking the deoxidization area formed within thefurnace in accordance with the control method shown in FIGS. 16 or 17,it is possible to maintain a concentration of Nitrogen Oxide at theoutlet 5 of the furnace to be always minimum.

FIG. 18 is a characteristic view which shows an example of a relationbetween the concentration of CO and the flow amount of the jet 18 fromthe gas port 6. Without the coal information 22 or the coal typemeasurement result 24, it is possible to mount a CO concentrationmeasuring apparatus 28 to, for example, the side wall 1 so as to takeinto a CO concentration signal 29 and control the flow amount of the jet18 from the gas port 6 in accordance with the concentration of CO. Inthis case, when the CO concentration signal 29 is equal to or more thanabout 4% as shown in FIG. 18, the damper 27 is opened so as to increasethe flow amount of the jet 18 at the gas port 6. In the case that theconcentration of CO 29 is equal to or less than 4%, the damper 27 isclosed so as to reduce the flow amount of the jet 18 at the gas port 6.As is apparent from the distribution of the concentration of CO shown inFIG. 9 mentioned above, it is not necessary to limit the concentrationof CO for starting the control to 4%. That is, when the concentration ofCO is equal to or less than 4% near the burners 2, 3 and 4, it isconsidered that a flame does not collide with the side wall 11 so thatit is possible to select an optional concentration of CO between 0 and4%.

Embodiment 5

FIGS. 19, 20 and 21 are side elevational views which show variations ofsupply means for supplying the air for jet 36 to a furnace 15.

The air for jet 36 shown in FIG. 19 is supplied by branching the air forcombustion 35 of the burner 39. Since the pressure of the air forcombustion 35 of the burner is high, it is possible to inject the jet 18at a high speed, so that it is preferable for increasing the pressurenear the side wall 1.

The air for jet 36 shown in FIG. 20 is branched from an upstream of thedamper 27 for adjusting the air flow amount of the burner 39. Whenbranching the air for jet 36 in a manner mentioned above, the pressureof the air for jet 36 is a little changed even by changing the flowamount of the air for combustion to the burner 39, so that it ispossible to inject the air for jet 36 at a further high speed. Further,it is possible to independently control the air for jet 36 and the airfor combustion in the burner 39.

FIG. 21 shows an embodiment in which in the case that the gas port 6 isclose to the after air port 9, the air for jet 36 is branched from theafter air 45 and the air pipe is made shorter.

In the conventional embodiment disclosed in Japanese Patent UnexaminedPublication No. 7-98103 mentioned above, the pipe for supplying the gasfor combustion having an oxygen partial pressure of 10% or less to theauxiliary combustion port was necessary. Accordingly, it is necessary toarrange the pipe for supplying the gas for combustion having a length ofsome tens meters, so that a large cost increase was unavoidable.

On the contrary, in the supply means for supplying the air for jet 36 tothe furnace 15 in accordance with the present invention as shown inFIGS. 19, 20 and 21, it is sufficient to only branch the air forcombustion 35 or the after air 45 piped to a very near position so as tosupply the air for jet 36. In particular, in the case that the gas port6 is provided at the same height as that of the burner stages 2, 3 and4, since it is possible to form the gas port 6 at both right and leftends of a window box 40 in the burner 39, it is sufficient to add only aminimum number of equipment for the present invention. In the case thatthe gas port 6 is provided at the same height as that of the after airport 9, the same matter can be also applied.

In accordance with the present invention, since in a once-through boilercomprising a combustion chamber formed by front and rear walls and aside wall crossing to said front and rear walls and a plural stages ofburners placed on at least one of said front and rear walls, a gas portis provided between an outermost row burner and said side wall within arange of a height of said burner stages so as to inject a gas into thecombustion chamber, thereby making a pressure of a gas near the sidewall higher than a pressure of a gas at a center portion of thecombustion chamber, it is possible to prevent the combustion gas fromcoming close to the side wall, thereby reducing an attachment of the ashdue to a collision of the combustion gas, a concentration of CO at anoutlet of the furnace and an unburned matter.

What is claimed is:
 1. A boiler comprising: a combustion chamber formedby front and rear walls and a side wall extending between said front andrear walls and plural stages of burners placed on at least one of saidfront and rear walls, at least one gas jet port in said at least one ofsaid front and rear walls for making a pressure of a gas near said sidewall within said combustion chamber higher than a pressure of a gas at acenter portion of said combustion chamber, said at least one gas jetport being at a height within a range of a height of said burner stages;opposing one s of said at least one gas jet port being provided in anopposing manner at the same height in the front and rear walls withinthe height range of the burners stages; the burner stages includingmeans for supplying pulverized coal as a fuel, air for transferring thepulverized coal and air for burning; means for supplying a part of theair for burning in a branched manner to the gas jet port and forinjecting said part of the air into the combustion chamber; and whereinthe air is inject ed from the at least one gas jet port in a form of adirect flow.
 2. A boiler as claimed in claim 1, further comprising saidopposing ones of said at least one gas jet port being respectivelyprovided at opposing portions of said front and rear walls, saidopposing portions being at the same height, and gas supply means forsupplying gas at a speed at which gas jets from said opposing gas portscollide midway between said front and rear walls.